The present invention generally relates to phase shifting circuits for pulse signals, and more particularly to a phase shifting circuit for outputting an output pulse signal by shifting the phase of an input pulse signal while maintaining the phase relationship between the input pulse signal and the output pulse signal substantially constant even when the period of the input pulse signal changes.
Generally, in a display device such as a television receiver which uses a picture tube, a starting point of the raster is determined by a synchronizing signal for both the horizontal and vertical directions on the screen. But in some cases, it is desirable to start the raster from a point which is shifted from the starting point determined by the synchronizing signal. For example, in the display device for a computer, it is necessary for the relative position between the synchronizing signal and the picture information to be freely adjustable since the time position relationship between the synchronizing signal and the picture displayed on a screen is different depending on the type of the computer.
In other words, it will be useful if it is possible to produce a new pulse signal which has the same period as the synchronizing signal and has a phase which leads or lags the phase of the synchronizing signal by a predetermined time, and supply this new pulse signal to the display device as the synchronizing signal so as to determine the starting point of the raster. In this case, the relative position of the picture information with respect to the new synchronizing signal changes, and it becomes possible to freely adjust the position of the picture on the screen. In the case where the synchronizing signal is a vertical synchronizing signal, for example, the picture can be moved upwardly and downwardly on the screen. On the other hand, the picture can be moved rightwardly and leftwardly in the case where the synchronizing signal is a horizontal synchronizing signal.
As an example of a conventional phase shifting circuit which shifts the phase of an input pulse signal, there is a phase shifting circuit comprising first and second monostable multivibrators and a shaping circuit. The input pulse signal is delayed by a predetermined time in the first monostable multivibrator and is further delayed by a predetermined delay time in the second monostable multivibrator. An output signal waveform of the second monostable multivibrator is shaped in the shaping circuit into an output pulse signal which has the same period and approximately the same pulse width as the input pulse signal but is delayed by a predetermined time. The phase of the output pulse signal can be freely changed to lead or lag with respect to the phase of the input pulse signal by adjusting a variable resistor coupled to the second monostable multivibrator so as to vary the output pulse width of the second monostable multivibrator. Accordingly, in the case where the input pulse signal is the synchronizing signal, the output pulse signal can be supplied to the display device as the new synchronizing signal which determines the starting point of the raster so as to move the picture in the vertical or horizontal direction of the screen.
In the case where the period of the synchronizing signal is constant as in the case of the general television system, no problems will occur when the phase of the synchronizing signal is shifted in the conventional phase shifting circuit. However, in the case where picture information sources having different horizontal deflection frequencies coexist, problems described hereunder occur when the phase of the synchronizing signal is shifted in the conventional phase shifting circuit. For example, picture information sources such as computers and high definition television systems have horizontal deflection frequencies in the range of 15 kHz to 30 kHz.
In other words, when the conventional phase shifting circuit having the output pulse widths of the first and second monostable multivibrators set in accordance with the period of a first synchronizing signal from a first picture information source receives from a second picture information source a second synchronizing signal having a period shorter than the period of the first synchronizing signal, the phase of the output pulse signal (output synchronizing signal) becomes greatly deviated from the second sychronizing signal unless the output pulse widths of the first and second monostable multivibrators are newly set in accordance with the short period of the second synchronizing signal. In an extreme case where the period of the second synchronizing signal is shorter than the output pulse width of the first monostable multivibrator, no output pulse is obtained from the first monostable multivibrator and the phase shifting circuit as a whole will become inoperative. Hence, in the display device which may receive various picture information having synchronizing signals of different frequencies, the phase shifting circuit will not operate correctly unless the output pulse widths of the first and second monostable multivibrators are adjusted every time the input picture information changes. For this reason, there are problems in that the circuit construction of the phase shifting circuit becomes complex and that it is troublesome to make the necessary adjustments every time the period of the input pulse signal changes.